Various types of turntable connections have been derived with some allowing more twist than others. In the attached drawings FIGS. 4, 5 and 6 illustrate prior art arrangements. Broadly speaking they can be separated into three types:
(A) Single oscillating turntables as shown in FIG. 4 allow the truck chassis to change angle with the trailer chassis along the longitudinal dirction (i.e. around a lateral axis) to cope with crests and hollows, but do not allow any sideways twisting in the lateral direction (i.e. around a longitudinal axis) to cope with changes in road camber or potholes.
(B) Double oscillating turntables as shown in FIG. 5 allow the truck and trailer chassis to twist both longitudinally and laterally at the same time. With double oscillating turntables the amount of lateral twist has to be restricted because the truck chassis offers no resistance to the trailer rolling sideways during cornering. While a double oscillating turntable works satisfactorily with trailers having a low centre of gravity it is not suitable for trailers with higher centres of gravity because as the trailer rolls in relation to the truck, the centre of gravity moves in the direction of the twist and makes the vehicle more unstable.
The problem is compounded in the case of a tank-trailer (e.g. a petrol tanker) since the trailer itself is essentially rigid and its chassis cannot twist to accommodate uneven road surfaces.
(C) A number of turntable designs have been developed to allow restricted lateral twist. One of the better types is known as a "kompensator" sliding turntable (see FIG. 6) and allows the turntable feet to slide sideways along an arc with a radius approximately equal to the distance from the centre of gravity of the trailer to the feet so that the trailer effectively twists about its centre of gravity without moving laterally in the direction of roll. This allows the trailer to twist with a limited degree of independence from the prime mover without leading to instability when cornering. The kompensator principle is at present a widely used type of turntable for semi-trailer tanker applications.
A disadvantage of the kompensator principle is that it relies on slides to carry the full weight of the trailer whilst sliding sideways along a cradle, thus creating friction and wearing both the slides and the cradle. This leads to excessive wear in the slides which then tend to stick and in some cases break apart so allowing the trailer to break away from the prime mover. A second disadvantage is that during cornering the tanker twisting about its axis creates a momentum which is passed onto the prime mover when the trailer reaches the limit of its freedom to twist (i.e. a bump-stop).
It is an object of the present invention to mitigate these disadvantages.